There have been efforts in the past to construct the necks of stringed musical instruments, primarily guitars, in such a way as to prevent the warping and bowing of the necks of these instruments, which is caused by the tension placed upon the necks by the tightening of the strings of the instruments. Although the problem exists to some degree in almost any stringed instrument, the problem is acute in the manufacture of electric guitars where the effect of string tension is severest. Efforts have been made in the past to develop methods for preventing such bending of the instrument necks. However, none of the past methods involve the simplicity of construction and the apparent success of the present invention in maintaining the necks of such instruments in as straight a position as this invention.
The use of adjustable torsion rods in stringed instrument necks is dictated by the need to counteract the force exerted by the strings upon the body and neck of the instrument. To the maximum extent possible, the torsion rod should also be able to restrict the degree of warpage associated with wooden necks due to change in moisture content and temperature. Maintaining proper neck geometry is necessary to insure maximum "playability", tuning and sound, especially near the body of the instrument where the effects of the misalignment of the instrument neck are most profound.
Virtually all stringed instruments use stretched strings tuned by varying tension which, when played, involve the shortening of the strings by pressing the string against the neck of the instrument at particular points. On most guitars and guitar-like instruments, the neck has an array of wires embedded in the neck to vary the effective length of the strings and consequently the sound generated by the string when it is played. These "frets" are normally arranged according to a mathematical pattern so that the strings generate particularly desired frequencies during playing of the instrument.
When string tension, string height, fret alignment and bridge position are all in proper relationship, the instrument is said to possess proper intonation. This intonation is lost when any of the relationships are altered by warpage of any of these components. If a wooden instrument neck warps, it changes the tension on the strings causing improperly tuned notes to be generated. In addition, once initial warpage occurs, the string tension, altered to compensate for the warpage, simply increases the tension which caused the initial warpage and thus generally accelerates the process.
Because string tension generally provides an upward tension component from the neck of the instrument, there is little likelihood of a convex warpage of the neck downward or towards the back of the instrument. The primary sources of warpage or bowing are a concave warping of the neck or a bowing, either to the left or the right.
Torsion rods are used to prevent the neck of the instrument from warping or assuming a concave shape. There are three primary designs currently in use to prevent this type of warping. The most common type consists of one or more threaded steel rods placed inside the neck of the instrument. One end of the rod is anchored in the neck and an adjusting nut is placed upon the other end of the rod so that as the nut is tightened, the tuner end of the neck is pulled down. Since the depth of the neck of a musical instrument from top to bottom is limited, the practical limit of this design is diminished because the angle at which this steel rod can be placed from one end of the neck to the other is very limited. Furthermore, this design has a very serious drawback in that in order to create the amount of force necessary to cause a downward motion of the neck of the instrument, the compression force placed upon the neck of the instrument is approximately ten times the force created to cause the downward motion. This severe compression of the neck may cause the neck to develop a series of buckles, especially near the body of the instrument. This rippling effect may actually create a worse problem in terms of playability and tone than the original warpage problem, thus, with this design, the cure may be worse than the disease. This compression may also cause the neck to curve in the lateral plane either to the right or left. This is a common problem since most stringed instruments use thicker strings on one side of the neck than on the other, causing unequal stress. The compression force of this existing design aggravates the tendency of the neck to curve to one side.
Furthermore, this particular design also tends to cause a spiraling or twisting of the neck. This result is possible whenever the grain of the wood used to construct the neck fails to be perfectly straight and symmetrical throughout the length of the neck. In this condition, the neck tends to twist along its axis causing different string clearance along the neck of the instrument. This also affects the playability of the instrument. The tension or compression caused by this particular design aggravates the spiraling problem.
The second existing design consists of a threaded rod placed in the body of the neck of the musical instrument anchored at one end with an adjusting nut on the other end. In this design, the neck must be internally grooved to accept the concave shape of the torsion rod. As the nut is tightened, the rod tends to straighten and cause the neck to straighten. Since the rod is already curved in relation to the neck, if the neck of the instrument happens to warp in the direction of the curve, this design accentuates the warpage as the nut is tightened. Similarly with the first device of the prior art, the compression caused by the tightening of this device creates the same difficulties associated with the first design, and in some cases to a greater extent.
The third common type of adjustable tension rod incorporates two parallel rods fastened together at one end thereof and placed within the neck of the instrument. At the other end of the torsion rods, the upper rod abuts against an enlarged, dished washer. The lower rod extends through the center of the dished washer and is threaded at the end. As the adjusting nut is tightened, the lower rod becomes shorter, causing the entire device to assume an arc. This design overcomes some of the compression problems of the other designs allowing only the force of the arc to be transferred to the neck of the instrument. However, with this design, the forces that cause torsional spiraling and lateral curvature to the right or left of the neck of the instrument remain unchecked. This third design is much more expensive and difficult to install, thusly increasing the price of the instrument.
Another design disclosed in U.S. Pat. No. 4,237,944 discloses a design of the latter type. This structure and design requires a complete redesign and manufacture of the neck of the stringed instrument, which is foreign to the manufacture of many stringed instruments and is not adapted or adaptable to the common manufacturing techniques of many of the stringed instruments to which the present invention is easily adaptable.